Add Command Line Option for Model Selection

[AntreasAntoniou]

Problem Statement:

The current implementation of Mentat does not allow users to choose which language model to use, including the more advanced GPT-4 and its 32K version.

Proposed Solution:

• Implement a command-line option, for example, --model, that lets users specify the language model they want to use.
• Include validation to ensure the specified model is supported.

Acceptance Criteria:

• A user can specify the model via the command line.
• The system validates the model name against the supported models and returns an error if it's not supported.

Priority: High

🛠️ This feature is key to user customization and leveraging the full potential of the latest language models.

[biobootloader]

You can choose the model, as documented here: https://github.com/AbanteAI/mentat/blob/main/docs/configuration.md

But it would be good to have a command line option as well!